feat: support Kimi K3 dynamic tool loading in system messages (#19579)

## Background

Kimi K3 users can now load complete function tools at specific points in
a conversation, reducing unnecessary tool context while preserving
conversation order and prompt caching.

## Summary

Added the exported MoonshotAISystemMessageProviderOptions type, K3
dynamic system-message serialization without content, shared MFJS tool
normalization, and actionable warnings with message omission for
unsupported models.

## Testing

Added converter, tool preparation, generate, stream, model-gating,
schema-normalization, runtime-validation, and compile-time public API
tests.

## End-to-end Validation

- Added and ran the focused Moonshot dynamic-tool-loading example
against the live Kimi K3 API.
- Confirmed the accepted request contained a mid-conversation `{ role:
'system', tools: [...] }` message without `content`.

## Documentation

Expanded the Moonshot provider documentation with typed usage, model
limitations, request-local persistence, prompt-cache, context-cost, and
security guidance.

## Related Issues

Fixes #19557

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Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>